A rotating electric field ion mass spectrometer (REFIMS) has an analyzer cell configured with an entrance end, four spaced-apart longitudinal walls to which time-dependent phased RF potentials are applied, and a detector at its target end. This type of REFIMS cell is described in detail in U.S. Pat. No. 5,726,448 issued on Mar. 10, 1998, to S. J. Smith and A. Chutjian, and is illustrated schematically in FIG. 1. The time-dependent RF potentials applied to the cell walls create an RF field which effectively rotates the ion beam within the cell. As the ions of the beam are rotated into a spiral path in the cell, the rotating RF field disperses the ion beam according to the mass-to-electrical charge (m/e) ratio and velocity distribution present in the ion beam. The ions of the beam are deflected angularly on the target detector, depending on the m/e, RF amplitude, and RF frequency. The detector counts the incident ions to determine the m/e and velocity distribution of ions in the beam, thereby providing a profile of the elemental constituents in the beam. One possible advantage of this type of device is that the spectral readout can be developed over a two-dimensional detector plane, which provides enhanced profile information for analysis as compared to the conventional one-dimensional (spot or line) spectral readouts. Further descriptions of this type of system are provided in: Clemmons, J. H., 1992, “Sounding rocket observations of precipitating ions in the morning auroral region”, Ph. D. dissertation, Univ. California, Berkeley, 135 pp; and Clemmons, J. H., and Herrero, F. A., 1998, “Mass spectroscopy using a rotating electric field”, Rev. Sci. Instruments 69, 2285-2291.
Unfortunately, the REFIMS device heretofore has had severe inherent problems relating to ion entrance angle and sensitivity that have made it practically unusable. The abrupt transition from free-space to the RF electric field between the grids requires that the ion entrance angle, offset, and timing coincide with the resonant helical path at an exact RF phase. Looked at in reverse, a resonant ion beam exiting the grids would travel out at a particular angle and offset radius, in contrast to the incident beam direction along the central longitudinal axis. Constructing a device with these limitations is possible, but the loss of sensitivity is remarkable. Only ions entering the chamber at the exact RF phase will resonate, all others are rejected, even if of the correct mass. If this tolerance is off by +/−1 degree, it means a sensitivity loss of 180 times, even before filtering takes place.